A polyvinyl acetal-based resin is generally obtained by acetalizing a polyvinyl alcohol-based resin (hereinafter, polyvinyl alcohol is abbreviated as PVA) obtained by saponification of a polyvinyl ester-based resin and is a polymer having a structural unit wherein a continuous vinyl alcohol structural unit of the PVA-based resin is acetalized with an aldehyde compound, an unreacted vinyl alcohol structural unit, and a vinyl acetate structural unit which is an unsaponified part of the PVA-based resin. Since such a polyvinyl acetal-based resin is excellent in toughness, dispersibility of inorganic and organic powders, adhesiveness to various materials, and transparency and is soluble in many organic solvents, the resin is used in a variety of use applications, e.g., a paint such as a wash primer, a protective paint, and a metallic paint, a printing ink such as a flexographic ink and s gravure ink, a printed board, a varnish for a coil wire, an adhesive, a ceramic binder, s binder for a magnetic tape and the like, an ink-jet medium, s coating material for textile printing, a glass intermediate film, and the like.
Such a polyvinyl acetal-based resin is frequently used as an organic solvent solution. However, recently, from the viewpoint of environmental protection, it is requested to reduce an amount of the organic solvent to be used and, also for the polyvinyl acetal-based resin, the use as a solution having a higher concentration is required. Therefore, a polyvinyl acetal-based resin showing a low solution viscosity when it is made into a solution having a high concentration, is desired and, for example, there has been proposed a polyvinyl acetal-based resin obtained by acetalizing a PVA-based resin having ethylene as a constitutional unit in a main chain (see e.g., Patent Document 1).
Moreover, as the organic solvent in the case where the polyvinyl acetal-based resin is used as a solution, a mixed solvent of an aromatic-based solvent such as toluene or xylene and an alcoholic solvent has preferably been used because a solution having a low viscosity and showing a small change in viscosity during storage is obtained. However, since the aromatic-based solvent becomes a large burden to the environment and has a large influence on health, the use thereof tends to be limited and it is desired that the polyvinyl acetal-based resin solution would be a solution with an alcoholic single solvent.
However, a usual polyvinyl acetal-based resin has an insufficient solubility to the alcoholic single solvent. For example, the polyvinyl acetal-based resin described in Patent Document 1 shows a good solubility to an aromatic-based/alcoholic mixed solvent solution but the solubility thereof to the alcoholic single solvent is insufficient.
On the other hand, a hydroxyl group in an unreacted vinyl alcohol structural unit derived from a PVA-based resin as a material largely influences a property of the polyvinyl acetal-based resin. The larger amount of the hydroxyl group the resin has, the more easily the resin crystallizes through a hydrogen bond to give a polyvinyl acetal-based resin excellent in high elastic modulus and toughness. Therefore, the polyvinyl acetal-based resin obtained from, as a material, a PVA-based resin having a high degree of saponification, i.e., a large amount of the hydroxyl group, is used in use applications where a high elastic modulus and toughness are required, such as a film, a coated film, and a glass intermediate film.
However, the polyvinyl acetal-based resin derived from such a PVA-based resin having a high degree of saponification has an insufficient solubility to an alcoholic single solvent in some cases and, even when dissolved, only a solution having a low transparency is obtained, so that it is difficult to apply the resin to a film or the like where transparency is required.    Patent Document 1: JP-A-2003-183325